How to Calculate the Right Chip Load

All good tools require some care in use to work to their best. In your wood shop, cutting edge maintenance is a variable cost that can be controlled by optimizing the chip load of your cutting tools.

The combination of three factors will affect the life of the cutting tools before renewal of the edge (either by replacing an insert knife, or having a fixed tool resharpened). These factors are:

1. The rotation speed of the tool.
2. The number of cutting edges on the tool.
3. The feed rate of the material past the cutting edges.

How Knives Dull

When an insert knife makes contact with wood, it generates significant friction and flash heat. This sudden and extreme heat acts to form corrosive compounds from resins, glues, etc. in the wood. This corrosive action breaks down a cutting edge to what we see, feel and hear as a dull tool. Processed wood products like plywood or MDF will contain more of these compounds than solid wood, but all timber will contain some.

While a tool is rotating and cutting, the wood chip which is created works as a heat sink, absorbing the heat from the cutting edge. Bigger chips can absorb more heat, substantially increasing the life of the cutting edge.

You may know that a rotating tool cutting against the feed direction will create a chip that starts thin as the knife edge starts the cut, and finishes thicker as the edge exits the cut. This chip load is measurable, and can be calculated. We use a formula that tells us the average thickness of a wood chip.

Chip Load Formula

Chip Load = Feed Rate / (RPM x # Wings)

Chip Load - A measure of the amount of material removed by each wing of a tool.Feed Rate - The rate at which the wood moves through the cutter.Wings - The number of cutting edges on the tool.

Adjusting Your Chip Load

Finding the truly ideal chip load involves considering whether it's a cross-grain or along-the-grain cut, the the species of wood being cut. It doesn't take a dedicated specialist to help you improve your chip load, though. A handy heuristic can help you improve your chip load and get close to optimal.

The easiest part of the equation for you to control is your material feed speed, whether you're feeding pieces manually or with a power feeder. Feed several test pieces through your cutter, increasing your feed speed every time. Once you get a piece where the surface finish is too rough for your needs, reduce the feed rate by 10%, and feed another piece.

If you're happy with the finish on this piece, you've found an improved chip load! Use this feed rate to protect your cutting edges for the longest possible time, and save money and time on knife changes.